1. Field of the Invention
The present invention relates to a layered catalyst composite useful for reducing contaminants in exhaust gas streams, especially gaseous streams containing sulfur oxide contaminants. More specifically, the present invention is concerned with improved catalysts of the type generally referred to as xe2x80x9cthree-way conversionxe2x80x9d catalysts. The layered catalysts trap sulfur oxide contaminants which tend to poison three-way conversion catalysts used to abate other pollutants in the stream. The layered catalyst composites of the present invention have a sulfur oxide absorbing layer before or above a nitrogen oxide absorbing layer. The sulfur oxide absorbing layer selectively and reversibly absorbs sulfur oxides over nitrogen oxides and alleviates sulfur oxide poisoning of the nitrogen oxide trap.
2. Related Art
Three-way conversion catalysts (xe2x80x9cTWCxe2x80x9d) have utility in a number of fields including the abatement of nitrogen oxides (xe2x80x9cNOXxe2x80x9d), carbon monoxide (xe2x80x9cCOxe2x80x9d), and hydrocarbon (xe2x80x9cHCxe2x80x9d) pollutants from internal combustion engines, such as automobile and other gasoline-fueled engines. Three-way conversion catalysts are polyfunctional because they have the ability to substantially simultaneously catalyze the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides. Emissions standards for nitrogen oxides, carbon monoxide, and unburned hydrocarbon contaminants have been set by various government agencies and must be met by new automobiles. In order to meet such standards, catalytic converters containing a TWC catalyst are located in the exhaust gas line of internal combustion engines. The catalysts promote the oxidation by oxygen in the exhaust gas of the unburned hydrocarbons and carbon monoxide and the reduction of nitrogen oxides to nitrogen. For example, it is known to treat the exhaust of engines with a catalyst/NOX sorbent which stores NOX during periods of lean (oxygen-rich) operation, and releases the stored NOX during the rich (relatively fuel-rich) periods of operation. During periods of rich operation, the catalyst component of the catalyst/NOX sorbent promotes the reduction of NOX to nitrogen by reaction of NOX (including NOX released from the NOX sorbent) with HC, CO, and/or hydrogen present in the exhaust.
TWC catalysts exhibiting good activity and long life comprise one or more platinum group metals, e.g., platinum, palladium, rhodium, ruthenium, and iridium. These catalysts are employed with a high surface area, refractory oxide support such as a high surface area alumina coating. The support is carried on a suitable carrier or substrate such as a monolithic carrier comprising a refractory ceramic or metal honeycomb structure, or refractory particles such as spheres or short, extruded segments of a suitable refractory material. The supported catalyst is generally used with a NOX storage (sorbent) component including alkaline earth metal oxides, such as oxides of Ca, Sr and Ba, alkali metal oxides such as oxides of K, Na, Li and Cs, and rare earth metal oxides such as oxides of Ce, La, Pr and Nd, see U.S. Pat. No. 5,473,887.
Sulfur oxide (xe2x80x9cSOxxe2x80x9d) contaminants present in an exhaust gaseous stream tend to poison and thereby inactivate TWC catalysts. SOX is a particular problem because it is generated by the oxidation of sulfur compound impurities often found in gasoline and diesel fuel. IWC catalysts employing NOX storage components tend to suffer from loss of long-term activity because of SOX poisoning of the NOX traps. NOX trap components also trap SOX and form very stable sulfates which require extreme conditions and a high fuel penalty to regenerate the trapping capacity of the NOX storage component. A guard or filter (e.g., alumina) may be placed before the TWC catalyst to protect the catalyst from SOX but these guards or filters often become saturated with SoX. Without valves, these guards require artificial engine cycles to desorb SOx by creating extended rich A/F period at elevated temperature. However, the SOx released under these conditions normally caused high H2S emission with unpleasant odor and to some extent poison the downstream NOX absorber.
High surface refractory metal oxides are often employed as a support for many of the catalytic components. For example, high surface area alumina materials, also referred to as xe2x80x9cgamma aluminaxe2x80x9d or xe2x80x9cactivated aluminaxe2x80x9d typically exhibit a BET (Brunauer, Emmett, and Teller) surface area in excess of 60 square meters per gram (xe2x80x9cm2/gxe2x80x9d), and often up to about 200 m2/g or more. Such activated alumina is usually a mixture of the gamma and delta phases of alumina, but may also contain substantial amounts of eta, kappa and theta alumina phases. Refractory metal oxides other than activated alumina may be utilized as a support for at least some of the catalytic components in a given catalyst. For example, bulk ceria, zirconia, alpha alumina and other materials are known for such use. Although many of these materials have a lower BET surface area than activated alumina, that disadvantage tends to be offset by the greater durability of the resulting catalyst.
Exhaust gas temperatures can reach 1000xc2x0 C. in a moving vehicle and such elevated temperatures can cause activated alumina, or other support material, to undergo thermal degradation with accompanying volume shrinkage especially in the presence of steam. During this degradation, the catalytic metal becomes occluded in the shrunken support medium with a loss of exposed catalyst surface area and a corresponding decrease in catalytic activity. U.S. Pat. No. 4,171,288 discloses a method to stabilize alumina supports against such thermal degradation by the use of materials such as zirconia, titania, alkaline earth metal oxides such as baria, calcia, or strontia, or rare earth metal oxides such as ceria, lanthana, and mixtures of two or more rare earth metal oxides.
U.S. Pat. Nos. 4,714,694, 4.727,052, and 4,708,946 disclose the use of bulk cerium oxide (ceria) to provide a refractory oxide support for platinum group metals other than rhodium. Highly dispersed, small crystallites of platinum on the ceria particles may be formed and stabilized by impregnation with a solution of an aluminum compound followed by calcination.
U.S. Pat. No. 3,993,572 discloses catalysts for promoting selective oxidation and reduction reactions. The catalyst contains platinum group metal, rare earth metal (ceria) and alumina components which may be supported on a relatively inert carrier such as a honeycomb.
U.S. Pat. No. 4,714,694 discloses a method of making a material which includes impregnating bulk ceria or a bulk ceria precursor with an aluminum compound and calcining the impregnated ceria to provide an aluminum stabilized ceria.
U.S. Pat. No. 4,808,564 discloses a catalyst for the purification of exhaust gases having improved durability which comprises a support substrate, a catalyst carrier layer formed on the support substrate and catalyst ingredients carried on the catalyst carrier layer. The catalyst carrier layer comprises oxides of lanthanum and cerium in which the molar fraction of lanthanum atoms to total rare earth atoms is 0.05 to 0.20 and the ratio of the number of the total rare earth atoms to the number of aluminum atoms is 0.05 to 0.25.
U.S. Pat. No. 4,367,162 discloses a three-way catalyst system which comprises a carrier having a substructure of refractory material in the form of a honeycomb structure and a porous layer of a powder formed on the surface thereof selected from the group consisting of a powder of zirconium oxide and a mixed powder of zirconium oxide powder with at least powder selected from the group consisting of alumina, alumina-magnesia spinel and cerium oxide, and a catalyst ingredient supported thereon consisting of cerium oxide and a metal selected from the group consisting of platinum, palladium, and mixtures thereof.
U.S. Pat. No. 4,438,219 discloses an alumina catalyst, stable at high temperatures, for use on a substrate. The stabilizing material is derived from barium, silicon, rare earth metals, alkali and alkaline earth metals, boron, thorium, hafnium, and zirconium. Barium oxide, silicon dioxide, and rare earth oxides including lanthanum, cerium, praseodymium, and neodymium are preferred. Contacting the stabilizing material with a calcined alumina film permits the calcined alumina film to retain a high surface area at higher temperatures.
U.S. Pat. Nos. 4,476,246, 4,591.578 and 4,591,580 disclose three-way catalyst compositions comprising alumina, ceria, an alkali metal oxide promoter, and Noble metals. U.S. Pat. Nos. 3,993,572 and 4,157,316 describe attempts to improve the catalyst efficiency of Pt/Rh based TWC systems by incorporating a variety of metal oxides, e.g., rare earth metal oxides such as ceria and base metal oxides such as nickel oxides. U.S. Pat. No. 4,591,518 discloses a catalyst comprising an alumina support with catalytic components consisting essentially of a lanthana component, ceria, an alkali metal oxide, and a platinum group metal. U.S. Pat. No. 4,591,580 discloses an alumina supported platinum group metal catalyst modified to include support stabilization by lanthana or lanthana rich rare earth oxides, double promotion by ceria and alkali metal oxides and optionally nickel oxide.
U.S. Pat. No. 4,624,940 discloses palladium containing catalyst compositions useful for high temperature applications. The combination of lanthanum and barium is found to provide a superior hydrothermal stabilization of alumina which supports the catalytic component, palladium. Thus, the palladium metal expulsion from the alumina due to phase transformation to encounter drastic sintering upon high temperature exposure is avoided. The use of particulate bulk metal oxide enhances catalytic activities. The bulk metal oxide consists of primarily ceria containing and/or ceria-zirconia containing particles. These particulate bulk metal oxides do not readily react with the stabilized alumina particles, thus, provide the catalytically promoting effect.
U.S. Pat. No. 4,780,447 discloses a catalyst capable of controlling HC, CO and NOX as well as H2S in emissions from the tailpipe of catalytic converter equipped automobiles. The use of nickel oxides and/or iron oxides is known as a H2S gettering of compound.
U.S. Pat. No. 4,294,726 discloses a TWC catalyst composition containing platinum and rhodium obtained by impregnating a gamma alumina carrier material with an aqueous solution of cerium, zirconium and iron salts or mixing the alumina with oxides of, respectively, cerium, zirconium and iron, and then calcining the material at 500xc2x0 C. to 700xc2x0 C. in air after which the material is impregnated with an aqueous solution of a salt of platinum and a salt of rhodium dried and subsequently treated in a hydrogen-containing gas at a temperature of 250xc2x0 C.-650xc2x0 C. The alumina may be thermally stabilized with calcium, strontium, magnesium or barium compounds. The ceria-zirconia-iron oxide treatment is followed by impregnating the treated carrier material with aqueous salts of platinum and rhodium and then calcining the impregnated material.
U.S. Pat. No. 4,965,243 discloses a method to improve the thermal stability of a TWC catalyst containing precious metals by incorporating a barium compound and a zirconium compound together with ceria and alumina to form a catalytic moiety to enhance stability of the alumina washcoat upon exposure to high temperature.
JP1210032 and AU-615721 disclose a catalytic composition comprising palladium, rhodium, active alumina, a cerium compound, a strontium compound and a zirconium compound. These patents suggests the utility of alkaline earth metals in combination with ceria, zirconia to form a thermally stable alumina supported palladium containing washcoat.
U.S. Pat. No. 4,504,598 discloses a process for producing a high temperature resistant TWC catalyst. The process includes forming an aqueous slurry of particles of gamma or activated alumina and impregnating the alumina with soluble salts of selected metals including cerium, zirconium, at least one of iron and nickel and at least one of platinum, palladium and rhodium and, optionally, at least one of neodymium, lanthanum, and praseodymium. The impregnated alumina is calcined at 600xc2x0 C. and then dispersed in water to prepare a slurry which is coated on a honeycomb carrier and dried to obtain a finished catalyst.
U.S. Pat. Nos. 3,787,560, 3,676,370, 3,552,913, 3,545,917, 3,524,721 and 3,899,444 disclose the use of neodymium oxide for use in reducing nitric oxide in exhaust gases of internal combustion engines. U.S. Pat. No. 3,899,444 in particular discloses that rare earth metals of the lanthanide series are useful with alumina to form an activated stabilized catalyst support when calcined at elevated temperatures. Such rare earth metals are disclosed to include lanthanum, ceria, cerium, praseodymium, neodymium and others.
U.S. Pat. No. 5,792,436 discloses a method for removing nitrogen oxides, sulfur oxides, and phosphorus oxides from a lean gaseous stream. The method comprises (a) passing the gaseous stream through a catalyzed trap comprising a regenerable sorbent material and an oxidation catalyst and sorbing the sorbable components into the sorbent material, (b) introducing a combustible component into the gaseous stream upstream of the catalyzed trap member and combusting the combustible component in the presence of the oxidation catalyst to thermally desorb the sorbable component from the sorbent material, and (c) passing the sorbable component-depleted stream to a catalytic treatment zone for the abatement of the pollutants and by-passing the sorbable component-enriched stream around the catalytic treatment zone.
TWC catalyst systems comprising a carrier and two or more layers of refractory oxide are disclosed. Japanese Patent Publication No. 145381/1975 discloses a catalyst-supported structure for purifying exhaust gases comprising a thermally insulating ceramic carrier and at least two layers of catalyst containing alumina or zirconia, the catalysts containing alumina or zirconia layers being different from each other.
Japanese Patent Publication No. 105240/1982 discloses a catalyst for purifying exhaust gases containing at least two carrier layers of a refractory metal oxide, each containing a different platinum-group metal. A layer of a refractory metal oxide free from the platinum-group metal is positioned between the carrier layers and/or on the outside of these carrier layers.
Japanese Patent Publication No. 52530/1984 discloses a catalyst having a first porous carrier layer composed of an inorganic substrate and a heat-resistant Noble metal-type catalyst deposited on the surface of the substrate and a second heat-resistant non-porous granular carrier layer having deposited thereon a Noble metal-type catalyst. The second carrier layer is formed on the surface of the first carrier layer and has resistance to the catalyst poison.
Japanese Patent Publication No. 127649/1984 discloses a catalyst for purifying exhaust gases comprising an inorganic carrier substrate such as cordierite, an alumina layer formed on the surface of the substrate and having deposited thereon a rare earth metal, such as lanthanum and cerium, and platinum or palladium, and a second layer formed on the first alumina-based layer and having deposited thereon a base metal such as iron or nickel and a rare earth metal such as lanthanum or rhodium.
Japanese Patent Publication No. 19036/1985 discloses a catalyst for purifying exhaust gases having an enhanced ability to remove carbon monoxide at low temperatures. The catalyst comprises a substrate composed of cordierite and two layers of active alumina laminated to the surface of the substrate. The lower alumina layer contains platinum or vanadium deposited thereon, and the upper alumina layer contains rhodium and platinum, or rhodium and palladium, deposited thereon.
Japanese Patent Publication No. 31828/1985 discloses a catalyst for purifying exhaust gases comprising a honeycomb carrier and a Noble metal having a catalytic action for purifying exhaust gases. The carrier is covered with an inside and an outside alumina layer, the inside layer having more Noble metal adsorbed thereon than the outside layer.
Japanese Patent Publication No. 232253/1985 discloses a monolithic catalyst for purifying exhaust gases in the shape of a pillar and comprising a number of cells disposed from an exhaust gas inlet side toward an exhaust gas outlet side. An alumina layer is formed on the inner wall surface of each of the cells and catalyst ingredients are deposited on the alumina layer. The alumina layer consists of a first alumina layer on the inside and a second alumina layer on the surface side, the first alumina layer having palladium and neodymium, and the second alumina layer having platinum and rhodium.
Japanese Kokai 71538/87 discloses a catalyst layer supported on a catalyst carrier and containing one catalyst component selected from the group consisting of platinum, palladium and rhodium. An alumina coat layer is provided on the catalyst layer. The coat layer contains one oxide selected from the group consisting of cerium oxide, nickel oxide, molybdenum oxide, iron oxide and at least one oxide of lanthanum and neodymium (1-10% by wt.).
U.S. Pat. Nos. 3,956,188 and 4,021,185 disclose a catalyst composition having (a) a catalytically active, calcined composite of alumina, a rare earth metal oxide and a metal oxide selected from the group consisting of an oxide of chromium, tungsten, a group IVB metal and mixtures thereof and (b) a catalytically effective amount of a platinum group metal added thereto after calcination of the composite. The rare earth metals include cerium, lanthanum and neodymium.
U.S. Pat. No. 4,806,519, discloses a two layer catalyst structure having alumina, ceria and platinum on the inner layer and aluminum, zirconium and rhodium on the outer layer.
JP-88-240947 discloses a catalyst composite which includes an alumina layer containing ceria, ceria-doped alumina and at least one component selected from the group of platinum, palladium and rhodium. A second layer contains lanthanum-doped alumina, praseodymium-stabilized zirconium, and lanthanum oxide and at least one component selected from the group of palladium and rhodium. The two layers are placed on a catalyst carrier separately to form a catalyst for exhaust gas purification.
Japanese Patent J-63-205141-A discloses a layered automotive catalyst in which the bottom layer comprises platinum or platinum and rhodium dispersed on an alumina support containing rare earth oxides, and a top coat which comprises palladium and rhodium dispersed on a support comprising alumina, zirconia and rare earth oxides.
Japanese Patent J-63-077544-A discloses a layered automotive catalyst having a first layer comprising palladium dispersed on a support comprising alumina, lanthana and other rare earth oxides and a second coat comprising rhodium dispersed on a support comprising alumina, zirconia, lanthana and rare earth oxides.
Japanese Patent J-63-007895-A discloses an exhaust gas catalyst comprising two catalytic components. One component comprises platinum dispersed on a refractory inorganic oxide support and a second component comprises palladium and rhodium dispersed on a refractory inorganic oxide support.
U.S. Pat. No. 4,587,231 discloses a method of producing a monolithic three-way catalyst for the purification of exhaust gases. A mixed oxide coating is applied to a monolithic carrier by treating the carrier with a coating slip in which an active alumina powder containing cerium oxide is dispersed together with a ceria powder and then baking the treated carrier. Platinum, rhodium and/or palladium are then deposited on the oxide coating by a thermal decomposition. Optionally, a zirconia powder may be added to the coating slip.
U.S. Pat. No. 4,134,860 relates to catalyst compositions that can contain platinum group metals, base metals, rare earth metals and refractory supports. The composition can be deposited on a relatively inert carrier such as a honeycomb. U.S. Pat. No. 4,923,842 discloses a catalytic composition for treating exhaust gases comprising a first support having dispersed thereon at least one oxygen storage component and at least one Noble metal component, and having dispersed immediately thereon an overlayer comprising lanthanum oxide and optionally a second support. The layer of catalyst is separate from the lanthanum oxide. The Nobel metal can include platinum, palladium, rhodium, ruthenium and iridium. The oxygen storage component can include the oxide of a metal from the group consisting of iron, nickel, cobalt and the rare earths. Illustrative of these are cerium, lanthanum, neodymium, praseodymium, etc.
U.S. Pat. No. 5,057,483 discloses a catalyst composition disposed in two discrete coats on a carrier. The first coat includes a stabilized alumina support on which a first platinum catalytic component and bulk ceria is dispersed, a bulk iron oxide, a metal oxide such as bulk nickel oxide (which is effective for the suppression of hydrogen sulfide emissions), and one or both of baria and zirconia dispersed throughout the first coat as a thermal stabilizer. The second coat, which may comprise a top coat overlying the first coat, contains a co-formed (e.g., co-precipitated) rare earth oxide-zirconia support on which a first rhodium catalytic component is dispersed, and a second activated alumina support having a second platinum catalytic component dispersed thereon. The second coat may also include a second rhodium catalytic component, and optionally, a third platinum catalytic component, dispersed as an activated alumina support.
U.S. Pat. No. 5,472,673 discloses an exhaust gas purification device for an engine. The device comprises an engine, an exhaust passage, an NOx absorbent, and a sulphur trapping means. The exhaust passage extends from an upstream end which receives exhaust gas from the engine to a downstream end from which exhaust gas is released. The NOx absorbent is arranged in the exhaust passage wherein the NOx absorbent absorbs NOx contained in the exhaust gas when a concatenation of oxygen in the exhaust gas flowing into the NOx absorbent is above a predetermined oxygen concentration. The NOx absorbent releases the absorbed NOx when the concentration of oxygen in the exhaust gas flowing into the NOx absorbent is lower than the predetermined oxygen concentration. The sulphur trapping means is arranged in the exhaust passage upstream of the NOx absorbent for trapping SOx contained in the exhaust gas wherein the trapped SOx is not released from the sulphur trapping means when the concentration of oxygen in the exhaust gas flowing into the sulphur trapping means is lower than the predetermined oxygen concentration so that SOx is prevented from reaching and being absorbed into the NOx absorbent.
U.S. Pat. No. 5,687,565 discloses a method for reducing the concentration of carbon monoxide, organic compounds and sulfur oxides in an exhaust gas from an internal combustion engine. The method comprises (a) contacting the exhaust gas with a sulfur oxide absorbent in a first contacting zone and absorbing with the sulfur oxide absorbent at least a portion of the sulfur oxides in the exhaust gas wherein the sulfur oxide absorption is substantially irreversible at temperatures which are less than or equal to that of the exhaust gas; (b) contacting the effluent gas from the first contacting zone with a catalyst in a second contacting zone and catalyzing the conversion of at least a portion of the carbon monoxide and organic compounds in the effluent gas from the first contacting zone to innocuous products; and (c) transferring heat from the exhaust gas to the second contacting zone by indirect heat exchange.
U.S. Pat. No. 5,687,565 discloses a system for exhaust gas purification disposed in an exhaust pipe of an internal combustion engine. The system comprises a catalyst composition giving an excellent light-off performance at low temperatures which comprises a precious metal and a substance having at least one of an electron donatability and a nitrogen dioxide absorbability and releasability, and optionally an adsorbent having hydrocarbon adsorbability.
WO92/09848 discloses a combustion catalyst comprising palladium and optionally a Group 1B or VIII noble metal which may be placed on a support comprising zirconium. The combustion catalyst may be graded to have a higher activity portion at the leading edge of the catalyst structure. The invention includes a partial combustion process in which the fuel is partially combusted using that catalyst. The catalyst structure is stable in operation, has a comparatively low operating temperature, has a low xe2x80x9clight offxe2x80x9d temperature, and is not susceptible to temperature xe2x80x9crunawayxe2x80x9d. The combustion gas produced by the catalytic process may be at a temperature below the autocombustive temperature, may be used at that temperature, or fed to other combustive stages for further use in a gas turbine, furnace, or boiler.
The conventional catalysts described above employing NOX storage components have the disadvantage under practical applications of suffering from long-term activity loss because of SOX poisoning of the NOX traps. The NOX trap components employed in the catalysts tend to trap SOX and form very stable sulfates which require extreme conditions and extract a high fuel penalty to regenerate the trapping capacity of the NOX storage component. Accordingly, it is a continuing goal to develop a three-way catalyst system which can reversibly trap SOX present in the gaseous stream and thereby prevent SOX sulfur oxide poisoning of the NOX trap.
The present invention relates to a thermally stable, layered catalyst composite of the type generally referred to as a three-way conversion catalyst (TWC). TWC catalysts are polyfunctional because they have the ability to substantially simultaneously catalyze the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides. The layered catalyst composites of the present invention have a sulfur oxide absorbing layer before or above a nitrogen oxide absorbing layer. The sulfur oxide absorbing layer selectively and reversibly absorbs sulfur oxides over nitrogen oxides and thereby alleviates sulfur oxide poisoning of the three-way conversion catalyst. Because SOX poisoning of the three-way conversion catalysts is minimized, the layered catalyst composites are able to maintain long term activity and effectively oxidize hydrocarbons and carbon monoxide and reduce nitrogen oxide compounds.
In a first embodiment, the structure of the layered catalyst composite of the present invention is designed in a radial arrangement wherein there is a first layer having a first layer composition and a second layer having a second layer composition. The first layer is also referred to as the bottom or inner layer and the second layer is referred to as the top or outer layer. Exhaust gaseous emissions comprising hydrocarbons, carbon monoxide, nitrogen oxides, and sulfur oxides initially encounter the second or top layer, and thereafter encounter the first or bottom layer. The top layer comprises a support and a SOX sorbent component having a free energy of formation from about 0 to about xe2x88x9290 Kcal/mole at 350xc2x0 C. The bottom layer comprises a support and a platinum component to catalyze the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides. The bottom layer may optionally include a NOX sorbent component selected from the group consisting of alkaline earth metal components, alkali metal components, and rare earth metal components. Upon passing through the top layer, the exhaust gas becomes depleted in SOX and then contacts the bottom layer. In the bottom layer, the three-way conversion catalyst/NOX sorbent stores NOX during lean periods and releases and reduces stored NOX during rich periods.
In use, the exhaust gas stream, which is contacted with the layered catalyst composite of the present invention, is alternately adjusted between lean and stoichiometric/rich operating conditions so as to provide alternating lean operating periods and stoichiometric/rich operating periods. The exhaust gas stream being treated may be selectively rendered lean or stoichiometric/rich either by adjusting the air-to-fuel ratio fed to the engine generating the exhaust or by periodically injecting a reductant into the gas stream upstream of the catalyst. For example, the layered catalyst composite of the present invention is well suited to treat the exhaust of engines, including diesel engines, which continuously run lean. In such case, in order to establish a stoichiometric/rich operating period, a suitable reductant, such as fuel, may be periodically sprayed into the exhaust immediately upstream of the catalytic trap of the present invention to provide at least local (at the catalytic trap) stoichiometric/rich conditions at selected intervals. Partial lean-burn engines, such as partial lean-burn gasoline engines, are designed with controls which cause them to operate lean with brief, intermittent rich or stoichiometric conditions. In practice, the SOX sorbent components in the top layer selectively absorb in-coming SOX during a lean mode operation (200xc2x0 C. to 600xc2x0 C.) and desorb SOX (regenerate) during a rich mode operation (450xc2x0 C. to 750xc2x0 C). When the exhaust gas temperature returns to a lean mode operation (200xc2x0 C. to 600xc2x0 C.), the regenerated SOX sorbent components in the top layer can again selectively absorb in-coming SOX. The duration of the lean mode may be controlled so that the SOX trap in the top layer will not be saturated with SOX. For example, a vehicle can run from 5 to 8 hours in a lean mode before a rich mode (60-100mile/hour running at stoichiometric or L=0.98) is required. The lean duration of the run is inversely proportional to the sulfur content in the fuel. The rich mode is preferred to be carried out at high-speed fuel-enrichment stage where engine cooling by fuel is a common practice.
In a preferred embodiment, the first layer of the layered catalyst composite comprises a first support, a first platinum component, optionally a first platinum group metal component other than platinum, and optionally a NOX sorbent component selected from the group consisting of alkaline earth metal components, alkali metal components, and rare earth metal components. The optional first platinum group metal component other than platinum in the first layer may be selected from the group consisting of palladium, rhodium, ruthenium, and iridium components. The preferred first platinum group metal component other than platinum in the first layer is selected from the group consisting of palladium, rhodium, and mixtures thereof. Preferably, the NOX sorbent component is selected from the group consisting of oxides of calcium, strontium, and barium, oxides of potassium, sodium, lithium, and cesium, and oxides of cerium, lanthanum, praseodymium, and neodymium. The first layer may additionally comprise a first zirconium component. Preferably, the first layer comprises at least one first alkaline earth metal component and at least one first rare earth metal component selected from the group consisting of lanthanum metal components and neodymium metal components.
In this preferred embodiment, the second layer of the layered catalyst composite comprises a second support and a SOX sorbent component having a free energy of formation from about 0 to about xe2x88x9290 Kcal/mole at 350xc2x0 C. The second layer may optionally comprise a second platinum component to facilitate NOX/SOx oxidization and NOX/SOX decomposition and reduction and optionally at least one second platinum group metal component other than platinum. The optional second platinum group metal component other than platinum in the second layer may be selected from the group consisting of palladium, rhodium, ruthenium, and iridium components. The preferred second platinum group metal component other than platinum in the second layer is selected from the group consisting of palladium, rhodium, and mixtures thereof. The second layer may additionally optionally comprise a second zirconium component. Preferably, the second layer comprises at least one second alkaline earth metal component and at least one second rare earth metal component selected from the group consisting of lanthanum metal components and neodymium metal components.
As set out above, the present invention employs a second or top layer of a SOX sorbent component which acts as a sulfur oxide absorbing layer to selectively and reversibly absorb sulfur oxides over nitrogen oxides and thereby provide a sulfur guard for the NOX trap component/three-way conversion catalyst. The SOX sorbent component in the SOX absorbing layer is a metal oxide which is less basic than the metal oxide in the NOX absorbing layer. The less basic SOx sorbent component forms SOX complexes (sulfates) that are less stable than the SOx complexes formed with the more basic NOX trap components. The SOX sorbent components of the present invention have a free energy of formation from about 0 to about xe2x88x9290 Kcal/mole at 350xc2x0 C., preferably from about 0 to about xe2x88x9260 Kcal/mole at 350xc2x0 C., and more preferably from about xe2x88x9230 to about xe2x88x9255 Kcal/mole at 350xc2x0 C. The free energy of formation is the free-energy change for a reaction in which a substance in its standard state is formed from its elements in their standard states. The free energy of a system is the internal energy of a system minus the product of its temperature and its entropy, that is G=Hxe2x88x92TS, where G is the Gibbs free energy, H is enthalpy, T is absolute temperature, and S is entropy. FIG. 1 shows the free energy of formation in Kcal/mole at 350xc2x0 C. for a number of metal oxides reacting to form nitrates, sulfates, carbonates, nitrites, and sulfites. In general, metals having a free energy of formation with NOX greater than about 0 Kcal/mole at 350xc2x0 C. (i.e., 10 Kcal/mole) will not readily adsorb NOX while metals having a free energy of formation with SOX lower than about xe2x88x9290 Kcal/mole at 350xc2x0 C. (i.e., xe2x88x92100 Kcal/mole) will form very stable sulfate but not readily desorb SOX.
FIG. 2 shows the free energy of formation in Kcal/mole at 350xc2x0 C., 650xc2x0 C., and 750xc2x0 C. for a number of metal oxides reacting to form nitrates and sulfates.
The top layer comprises SOX absorbing components which will not substantially absorb NOX under the operating conditions, e.g., from about 300xc2x0 C. to about 600xc2x0 C. The medium temperature regeneration SOX traps selectively absorb SOX so that the SOX traps will not be saturated with nitrate salts in the lean mode and consequently lose their SOX-trap capacity. The SOX sorbent component is capable of selectively absorbing SOX over NOX in a temperature range from about 100xc2x0 C. to about 600xc2x0 C. and capable of desorbing SOX in a temperature range from about 500xc2x0 C. to about 700xc2x0 C. Preferably, the SOX sorbent component is capable of selectively absorbing SOX over NOX in a temperature range from about 150xc2x0 C. to about 475xc2x0 C., more preferably in a temperature range from about 200xc2x0 C. to about 450xc2x0 C., and most preferably in a temperature range from about 250xc2x0 C. to about 450xc2x0 C. Preferably, the SOX sorbent component is capable of desorbing SOX over NOX in a temperature range from about 500xc2x0 C. to about 700xc2x0 C., preferably in a temperature range from about 520xc2x0 C. to about 658xc2x0 C., more preferably in a temperature range from about 535xc2x0 C. to about 675xc2x0 C., and most preferably in a temperature range from about 550xc2x0 C. to about 650xc2x0 C. Nonlimiting illustrative examples of SOX sorbent components may be selected from the group consisting of oxides and aluminum oxides of lithium, magnesium, calcium, manganese, iron, cobalt, nickel, copper, zinc, and silver. More preferred SOX sorbent components may be selected from the group consisting of MgO, MgAl2O4 (or hydrotalcite with MgO/Al2O3 from 9/1 to 1/9), MnO, MnO2, and Li2O. The most preferred SOX sorbent components are MgO and Li2O.
The thickness of the SOX absorbing layer is sufficiently dense and thick so as to create a SOX diffusion barrier or SOX sink to protect the bottom NOx absorbing layer from contacting SOX. The optimum thickness may vary with cpsi (cell density and wall thickness) of the substrates. Preferably, the SOX absorbing layer should be from about 0.3 g/in3 to about 2.4 g/in3 in loading, more preferably from about 0.8 g/in3 to about 1.8 g/in3.
The first and second supports may be the same or different compounds and may be selected from the group consisting of silica, alumina, and titania compounds. Preferably the first and second supports are activated compounds selected from the group consisting of alumina, silica, silica-alumina, alumino-silicates, alumina-zirconia, alumina-chromia, and alumina-ceria. More preferably, the first and second supports are activated alumina.
The first layer and second layer compositions may optionally comprise first and second alkaline earth metals which are believed to stabilize the first and second layer compositions, respectively. The first and second alkaline earth metal may be selected from the group consisting of magnesium, barium, calcium and strontium, preferably strontium and barium. Most preferably, the first alkaline earth metal component comprises barium oxide and the second alkaline earth metal component comprises strontium oxide. Stabilization means that the conversion efficiency of the catalyst composition of each layer is maintained for longer period of time at elevated temperatures. Stabilized supports such as alumina and catalytic components such as Noble metals are more resistant to degradation against high temperature exposure thereby maintaining better overall conversion efficiencies.
The first layer and second layer compositions may also optionally comprise first and second rare earth metal components which are believed to act as promoters. The rare earth metal components are derived from a metal selected from the group consisting of lanthanum and neodymium. In a specific embodiment, the first rare earth metal component is substantially lanthana and the second rare earth component is substantially neodymia. The promoter enhances the conversion of the hydrocarbons, carbon monoxide, nitrogen oxides, and sulfur oxides to harmless compounds. Zirconium component in both layers act as both washcoat stabilizer and promoter.
The first layer and second layer compositions may further comprise nickel, manganese, or iron components useful to remove sulfides such as hydrogen sulfides emissions. Most preferably, the first layer comprises a nickel, manganese, or iron compound.
Preferably, the first layer comprises a first support, a first platinum component, a first platinum group metal component other than platinum, and a NOx sorbent component selected from the group consisting of alkaline earth metal components, alkali metal components, and rare earth metal components, and a first zirconium component. Preferably, the second layer comprises a second support, a SOX sorbent component having a free energy of formation from about 0 to about xe2x88x9290 Kcal/mole at 350xc2x0 C., a second platinum component, at least one second platinum group metal component other than platinum, and a second zirconium component. Preferably, at least one of the first or second layers comprises at least one first alkaline earth metal component and at least one first rare earth metal component selected from the group consisting of lanthanum metal components and neodymium metal components.
When the compositions are applied as a thin coating to a monolithic carrier substrate, the proportions of ingredients are conventionally expressed as grams of material per cubic inch (g/in3) of the catalyst and the substrate. This measure accommodates different gas flow passage cell sizes in different monolithic carrier substrates. Platinum group metal components are based on the weight of the platinum group metal.
A useful and preferred first layer has from about 0.15 g/in3 to about 2.0 g/in3 of the first support; (ii) at least about 1 g/ft3 of the first platinum component; (iii) at least about 1 g/ft3 of a first platinum group metal component other than platinum; (iv) from about 0.025 g/in3 to about 0.5 g/in3 of a NOX sorbent component selected from the group consisting of alkaline earth metal oxides, alkali metal oxides, and rare earth metal oxides; and (v) from about 0.025 g/in3 to about 0.5 g/in3 of a first zirconium component; and from 0.0 and preferably about 0.025 g/in3 to about 0.5 g/in3 of at least one first rare earth metal component selected from the group consisting of ceria metal components, lanthanum metal components and neodymium metal component.
A useful and preferred second layer has from about 0.15 g/in3 to bout 2.0 g/in3 of the second support; (ii) from about 0.3 g/in3 to about 1.8 g/in3 of he SOX sorbent component; (iii) at least about 1 g/ft3 of a second platinum group component; (iv) at least about 1 g/ft3 of a second platinum group metal component other than platinum; and (v) from about 0.025 g/in3 to about 0.5 g/in3 of a second zirconium component.
The specific construction of layers having the first and second compositions in the layered catalyst composites set out above results in an effective three-way catalyst that reversibly traps sulfur oxide contaminants present and thereby prevents the sulfur oxide contaminants from poisoning the three-way conversion catalysts. The layered catalyst composite can be in the form of a self-supported article such as a pellet with the first layer on the inside and the second layer on the outside of the pellet. Alternatively, and more preferably, the first layer is supported on a carrier, also referred to as a substrate, preferably a honeycomb substrate, and the second layer is supported on the first layer applied to the substrate.
In a second embodiment, the structure of the layered catalyst composite of the present invention is designed in an axial arrangement wherein there is an upstream section and a downstream section. The upstream section comprises an upstream substrate and a first layer on the upstream substrate. The downstream section comprises a downstream substrate and a first layer on the downstream substrate. Exhaust gaseous emissions comprising hydrocarbons, carbon monoxide, nitrogen oxides, and sulfur oxides first encounter the upstream section, and secondly encounter the downstream section. The first layer comprises a first support; at least one first platinum component; optionally a first platinum group metal component other than platinum; optionally a first zirconium component; and optionally a NOX sorbent component selected from the group consisting of alkaline earth metal components, alkali metal components, and rare earth metal components. The second layer comprises a second support; a SOX sorbent component having a free energy of formation from about 0 to about xe2x88x9290 Kcal/mole at 350xc2x0 C.; optionally at least one second platinum component; optionally a second platinum group metal component other than platinum; and optionally a second zirconium component. Upon passing through the upstream section, the exhaust gas becomes depleted in SOX and then contacts the downstream section. In the downstream section, the three-way conversion catalyst/NOX sorbent stores NOX during lean periods and releases and reduces stored NOX during rich periods.
The layered catalyst composites of the present invention may also comprise several layers of several different basic metal oxide components which may be designed in a radial arrangement or an axial arrangement. In this embodiment, the less basic metal oxide components are utilized in the top layers or upstream sections and the more basic metal oxide components are utilized in the bottom layers or downstream sections to provide an alkaline gradient of basic metal oxides. The top layers or upstream sections serve mainly to absorb SOX and the bottom layers or downstream sections serve to absorb NOX.
In a specific second embodiment, the present invention pertains to an axial layered catalyst composite comprising an upstream section and a downstream section:
(1) the downstream section comprising:
(a) a downstream substrate; and
(b) a first layer on the downstream substrate, the first layer comprising a first support and a first platinum component;
(2) the upstream section comprising:
(a) an upstream substrate; and
(b) a second layer on the upstream substrate, the second layer comprising a second support and a SOX sorbent component having a free energy of formation from about 0 to about xe2x88x9290 Kcal/mole at 350xc2x0 C.
In a third embodiment, the present invention is directed to a radial layered catalyst composite comprising a bottom, a first middle, and a top layer. Exhaust gaseous emissions comprising hydrocarbons, carbon monoxide, nitrogen oxides, and sulfur oxides first encounter the top layer, secondly the first middle layer, and thirdly the bottom layer. The bottom layer comprises a first support; at least one first platinum component; a first NOX sorbent component selected from the group consisting of cesium components, potassium components, and cerium components; optionally a first platinum group metal component other than platinum; and optionally a first zirconium component. The first middle layer comprises a second support; at least one second metal oxide which is selected from the group consisting of BaO and MgO; optionally a second platinum component; optionally a second platinum group metal component other than platinum; and optionally a second zirconium component. The top layer comprises a third support; at least one third metal oxide component which is MgAl2O4; optionally a third platinum component; optionally a third platinum group metal component other than platinum; and optionally a third zirconium component. In one embodiment, the second metal oxide in the first middle layer is BaO. In another embodiment, the second metal oxide in the first middle layer is MgO. The NOX sorbent component in the bottom layer is preferably a composite of Cs2O/K2O/CeO2.
In this third embodiment, preferably the first middle layer comprises a SOx sorbent component which is MgO. Preferably, the radial layered catalyst composite further comprises a second middle layer located between the bottom layer and the first middle layer. Exhaust gaseous emissions comprising hydrocarbons, carbon monoxide, nitrogen oxides, and sulfur oxides first encounter the top layer, then the first middle layer, next the second middle layer, and finally the bottom layer. The second middle layer comprises a fourth support; and a fourth metal oxide which is BaO; optionally a fourth platinum component; optionally a fourth platinum group metal component other than platinum; and optionally a fourth zirconium component.
In specific third embodiment, the present invention pertains to a radial layered catalyst composite comprising a bottom layer, a first middle layer, and a top layer:
(a) the bottom layer comprising:
(i) a first support;
(ii) a first platinum component;
(iii) a first NOX sorbent component selected from the group consisting of cesium components, potassium components, and cerium components; and
(b) the first middle layer comprising:
(i) a second support;
(ii) a second SOX sorbent component which is selected from the group consisting of BaO and MgO; and
(c) the top layer comprising:
(i) a third support;
(ii) a third SOX sorbent component which is MgAl2O4.
Preferably, the radial layered catalyst composite in this embodiment further includes the following:
(3) the first middle layer comprises a SOX sorbent component which is MgO; and further comprising a second middle layer located between the bottom layer and the first middle layer:
(d) the second middle layer comprising:
(i) a fourth support; and
(ii) a SOX sorbent component which is BaO.
In a fourth embodiment, the present invention is directed to an axial layered catalyst composite having an upstream section, a midstream section, and a downstream section. Exhaust gaseous emissions comprising hydrocarbons, carbon monoxide, nitrogen oxides, and sulfur oxides first encounter the upstream section, then the midstream section, and finally the downstream section. The downstream section comprises a downstream substrate and a first layer on the downstream substrate. The first layer comprises a first support; at least one first platinum component; at least one NOX sorbent component which is selected from the group consisting of cesium components, potassium components, and cerium components; optionally a first platinum group metal component other than platinum; and optionally a first zirconium component. The upstream section comprises an upstream substrate and a second layer on the upstream substrate. The second layer comprises a second support; a SOx sorbent component which is MgAl2O4; optionally a second platinum component; optionally a second platinum group metal component other than platinum; and optionally a second zirconium component. The first midstream section, located between the upstream section and the downstream section, comprises a first midstream substrate and a third layer on the first midstream substrate. The third layer comprises a third support; a third metal oxide which is selected from the group consisting of BaO and MgO; optionally a third platinum component; optionally a platinum group metal component other than platinum; and optionally a third zirconium component. In one embodiment, the third metal oxide in the third layer is BaO. In another embodiment, the third metal oxide in the third layer is MgO. The NOX sorbent component in the first layer is preferably a composite of Cs2O/K2O/CeO2.
In this fourth embodiment, preferably the third layer on the first midstream substrate of the axial layered catalyst composite comprises a third metal oxide component which is MgO. Preferably, the axial layered catalyst composite further comprises a second midstream section located between the downstream section and the first midstream section. Exhaust gaseous emissions comprising hydrocarbons, carbon monoxide, nitrogen oxides, and sulfur oxides first encounter the upstream section, then the first midstream section, next the second midstream section, and finally the downstream section. The second midstream section comprises a second midstream substrate and a fourth layer on the second midstream substrate. The fourth layer comprises a fourth support; a fourth metal oxide which is BaO; optionally a fourth platinum component; optionally a fourth platinum group metal component other than platinum; and optionally a fourth zirconium component.
Preferably, the axial layered catalyst composite in this embodiment further includes the following:
(1) the first layer on the downstream substrate further comprises a NOx sorbent component selected from the group consisting of cesium components, potassium components, and cerium components; and
(2) the second layer on the upstream substrate comprises a SOX sorbent component which is MgAl2O4; and further comprising a first midstream section located between the upstream section and the downstream section:
(3) the first midstream section comprising:
(a) a first midstream substrate; and
(b) a third layer on the first midstream substrate, the third layer comprising:
(i) a third support; and
(ii) a third SOX sorbent component which is selected from the group consisting of BaO and MgO.
More preferably, the axial layered catalyst composite in this embodiment further includes the following:
(1) the third layer on the first midstream substrate comprises a third SOX sorbent component which is MgO; and further comprising a second midstream section located between the downstream section and the first midstream section:
(2) the second midstream section comprising:
(a) a second midstream substrate; and
(b) a fourth layer on the second midstream substrate, the fourth layer comprising:
(i) a fourth support;
(ii) a fourth SOX sorbent which is BaO.
The front or upstream longitudinal portion of the axial layered catalyst composite, the portion end to which the exhaust stream being treated is first introduced, preferably excludes the NOX sorbents which, when used, are relegated to a rear or downstream portion of the axial layered catalyst composite. For example, a typical so-called honeycomb-type carrier member comprises a xe2x80x9cbrickxe2x80x9d of material such as cordierite or the like, having a plurality of fine, gas-flow passages extending from the front portion to the rear portion of the carrier member. These fine gas-flow passages, which may number from about 100 to 900 passages or cells per square inch of face area (xe2x80x9ccpsixe2x80x9d), have a catalytic trap material coated on the walls thereof. Preferably, the NOX sorbent is utilized on the rear longitudinal segment of the carrier member so as to prevent the sulfur oxide contaminants from poisoning the three-way conversion catalysts. Typically, the first (front or upstream) 80% to 20% of the longitudinal length of the carrier member is kept substantially free of the NOX sorbents which are relegated to the rear 20% to 80% of the length of the catalytic trap. The same effect may be attained by using two separate carrier members in series, the first or upstream member being devoid of NOX sorbents, which may be contained in a second or downstream carrier member.
The present invention also includes a method for treating an exhaust gas stream which comprises the step of contacting the gas stream comprising carbon monoxide and/or hydrocarbons, nitrogen oxides, and sulfur oxides with the layered catalyst composite set out above. The present invention also includes a method of treating an exhaust gas stream comprising the steps of contacting the stream with the layered catalyst composite set out above under alternating periods of lean and stoichiometric or rich operation. Contacting is carried out under conditions whereby at least some of the SOX in the exhaust gas stream is trapped in the catalytic material during the periods of lean operation and is released and reduced during the periods of stoichiometric or rich operation.
In a specific embodiment, the present invention pertains to a method for removing NOX and SOX contaminants from a gaseous stream comprising the steps of:
(A) in a sorbing period, passing a lean gaseous stream within a sorbing temperature range through a layered catalyst composite comprising a first layer and a second layer:
(a) the first layer comprising a first support and a first platinum component; and
(b) the second layer comprising a second support and a SOX sorbent component having a free energy of formation from about 0 to about xe2x88x9290 Kcal/mole at 350xc2x0 C.; to sorb at least some of the SOX contaminants into the second layer and thereby provide a SOX depleted gaseous stream exiting the second layer and entering the first layer, wherein the first layer sorbs and abates the NOX in the gaseous stream; and
(B) in a desorbing period, converting the lean gaseous stream to a rich gaseous stream and raising the temperature of the gaseous stream to within a desorbing temperature range to thereby reduce and desorb at least some of the SOX contaminants from the second layer and thereby provide a SOX enriched gaseous stream exiting the second layer, preferably at high VHSV (space velocity)to reduce contact time of SOx with downstream catalytic layers.
In another specific embodiment, the present invention pertains to a method for removing NOX and SOX contaminants from a gaseous stream comprising the steps of:
(A) in a sorbing period, passing a lean gaseous stream within a sorbing temperature range through an axial layered catalyst composite comprising an upstream section and a downstream section:
(1) the downstream section comprising:
(a) a downstream substrate; and
(b) a first layer on the downstream substrate, the first layer comprising a first support and a first platinum component;
(2) the upstream section comprising:
(a) an upstream substrate; and
(b) a second layer on the upstream substrate, the second layer comprising a second support and a SOX sorbent component having a free energy of formation from about 0 to about xe2x88x9290 Kcal/mole at 350xc2x0 C.; to sorb at least some of the SOX contaminants into the upstream section and thereby provide a SOX depleted gaseous stream exiting the upstream section and entering the downstream section, wherein the downstream section sorbs and abates the NOX in the gaseous stream; and
(B) in a desorbing period, converting the lean gaseous stream to a rich gaseous stream and raising the temperature of the gaseous stream to within a desorbing temperature range to thereby reduce and desorb at least some of the SOx contaminants from the upstream section and thereby provide a SOX enriched gaseous stream exiting the upstream section, preferably at high VHSV (space velocity)to reduce contact time of SOx with downstream catalytic layers.
More preferably, the method in this embodiment further includes the following:
(1) the first layer on the downstream substrate further comprises a NOx sorbent component selected from the group consisting of cesium components, potassium components, and cerium components; and
(2) the second layer on the upstream substrate comprises a SOX sorbent component which is MgAl2O4; and further comprising a first midstream section located between the upstream section and the downstream section:
(3) the first midstream section comprising:
(a) a first midstream substrate; and
(b) a third layer on the first midstream substrate, the third layer comprising:
(i) a third support; and
(ii) a third SOX sorbent component which is selected from the group consisting of BaO and MgO; to sorb at least some of the SOX contaminants into the first midstream section and thereby provide a SOX depleted gaseous stream exiting the first midstream section and entering the downstream section, wherein the downstream section sorbs and abates the NOX in the gaseous stream; and
(B) in a desorbing period, converting the lean gaseous stream to a rich gaseous stream and raising the temperature of the gaseous stream to within a desorbing temperature range to thereby reduce and desorb at least some of the SOx contaminants from the first midstream section and thereby provide a SOX enriched gaseous stream exiting the first midstream section.
In yet another specific embodiment, the present invention pertains to a method for removing NOX and SOX contaminants from a gaseous stream comprising the steps of:
(A) in a sorbing period, passing a lean gaseous stream within a sorbing temperature range through a radial layered catalyst composite comprising a bottom layer, a first middle layer, and a top layer:
(a) the bottom layer comprising:
(i) a first support;
(ii) a first platinum component;
(iii) a first NOX sorbent component selected from the group consisting of cesium components, potassium components, and cerium components; and
(b) the first middle layer comprising:
(i) a second support;
(ii) a second SOX sorbent component which is selected from the group consisting of BaO and MgO; and
(c) the top layer comprising:
(i) a third support;
(ii) a third SOX sorbent component which is MgAl2O4; to sorb at least some of the SOX contaminants into the top and first middle layers and thereby provide a SOX depleted gaseous stream exiting the top and first middle layers and entering the bottom layer, wherein the bottom layer sorbs and abates the NOX in the gaseous stream; and
(B) in a desorbing period, converting the lean gaseous stream to a rich gaseous stream and raising the temperature of the gaseous stream to within a desorbing temperature range to thereby reduce and desorb at least some of the SOx contaminants from the top and first middle layers and thereby provide a SOX enriched gaseous stream exiting the top and first middle layers, preferably at high VHSV (space velocity)to reduce contact time of Sox with downstream catalytic layers.
More preferably, the method in this embodiment further includes the following:
(3) the first middle layer comprises a SOX sorbent component which is MgO; and further comprising a second middle layer located between the bottom layer and the first middle layer:
(d) the second middle layer comprising:
(i) a fourth support; and
(ii) a SOX sorbent component which is BaO; to sorb at least some of the SOX contaminants into the second middle layer and thereby provide a SOX depleted gaseous stream exiting the second middle layer and entering the bottom layer, wherein the bottom layer sorbs and abates the NOX in the gaseous stream; and
(B) in a desorbing period, converting the lean gaseous stream to a rich gaseous stream and raising the temperature of the gaseous stream to within a desorbing temperature range to thereby reduce and desorb at least some of the SOX contaminants from the second middle layer and thereby provide a SOX enriched gaseous stream exiting the second layer.
The present invention also includes a method for preparing the layered catalyst composite of the present invention which involves forming the first and second layers and then coating the first layer with the second layer. The present invention further includes a method of forming a layered catalyst composite which comprises the steps of (a) combining at least one water-soluble or dispersible first platinum component and a finely divided, high surface area refractory oxide with an aqueous liquid to form a first solution or dispersion which is sufficiently dry to absorb essentially all of the liquid; (b) optionally mixing the first solution or dispersion with a first water-soluble or dispersible platinum group metal component other than a platinum component, a first zirconium component, and a NOX sorbent component selected from the group consisting of alkaline earth metal components, alkali metal components, and rare earth metal components; (c) forming a first layer of the first solution or dispersion on a substrate; (d) converting the first platinum component and the optional first platinum group metal component other than platinum in the resulting first layer to a water-insoluble form (either by heat or pH change); (e) combining at least one water-soluble or dispersible SOx sorbent component, capable of selectively absorbing SOX over NOX in a temperature range from about 100xc2x0 C. to about 600xc2x0 C. and capable of desorbing SOX in a temperature range from about 500xc2x0 C. to about 700xc2x0 C., and a finely divided, high surface area refractory oxide with an aqueous liquid to form a second solution or dispersion which is sufficiently dry to absorb essentially all of the liquid; (f) optionally mixing the second solution or dispersion with a water-soluble or dispersible second platinum component, second platinum group metal component other than platinum, and a second zirconium component; (g) forming a second layer of the second solution or dispersion on the first layer; and (h) converting the second platinum component and the optional second platinum group metal component other than platinum in the resulting second layer to a water-insoluble form.
In a specific embodiment, the present invention pertains to a method of forming a layered catalyst composite which comprises the steps of:
(a) forming a first layer comprising:
(i) a first support; and
(ii) a first platinum component; and
(b) coating the first layer with a second layer comprising:
(i) a second support; and
(ii) a SOX sorbent component having a free energy of formation from about 0 to about xe2x88x9290 Kcal/mole at 350xc2x0 C.
In another specific embodiment, the present invention pertains to a method of forming a layered catalyst composite which comprises the steps of:
(a) combining a water-soluble or dispersible (a suspension of) first platinum component and a finely divided, high surface area refractory oxide with an aqueous liquid to form a first solution or dispersion which is sufficiently dry to absorb essentially all of the liquid;
(b) forming a first layer of the first solution or dispersion on a substrate;
(c) converting the first platinum component in the resulting first layer to a water-insoluble form;
(d) combining a water-soluble or dispersible SOX sorbent component having a free energy of formation from about 0 to about xe2x88x9290 Kcal/mole at 350xc2x0 C., and a finely divided, high surface area refractory oxide with an aqueous liquid to form a second solution or dispersion which is sufficiently dry to absorb essentially all of the liquid;
(e) forming a second layer of the second solution or dispersion on the first layer; and
(f) converting the second platinum component in the resulting second layer to a water-insoluble form.
As used herein, the following terms, whether used in singular or plural form, have the meaning defined below.
The term xe2x80x9ccatalytic metal componentxe2x80x9d, or xe2x80x9cplatinum metal componentxe2x80x9d, or reference to a metal or metals comprising the same, means a catalytically effective form of the metal or metals, whether the metal or metals are present in elemental form, or as an alloy or a compound, e.g., an oxide.
The term xe2x80x9ccomponentxe2x80x9d or xe2x80x9ccomponentsxe2x80x9d as applied to NOX sorbents means any effective NOX-trapping forms of the metals, e.g., oxygenated metal compounds such as metal hydroxides, mixed metal oxides, metal oxides or metal carbonates.
The term xe2x80x9cgaseous streamxe2x80x9d or xe2x80x9cexhaust gas streamxe2x80x9d means a stream of gaseous constituents, such as the exhaust of an internal combustion engine, which may contain entrained non-gaseous components such as liquid droplets, solid particulates, and the like.
The terms xe2x80x9cg/in3 xe2x80x9d or xe2x80x9cg/ft3xe2x80x9d or xe2x80x9cg/ft3xe2x80x9d used to describe weight per volume units describe the weight of a component per volume of catalyst or trap member including the volume attributed to void spaces such as gas-flow passages.
The term xe2x80x9cleanxe2x80x9d mode or operation of treatment means that the gaseous stream being treated contains more oxygen that the stoichiometric amount of oxygen needed to oxidize the entire reductants content, e.g., HC, CO and H2, of the gaseous stream.
The term xe2x80x9cmixed metal oxidexe2x80x9d means bimetallic or multi-metallic oxygen compounds, such as Ba2SrWO6, which are true compounds and is not intended to embrace mere mixtures of two or more individual metal oxides such as a mixture of SrO and BaO.
The term xe2x80x9cplatinum group metalsxe2x80x9d means platinum, rhodium, palladium, ruthenium, iridium, and osmium.
The term xe2x80x9cselectively absorbing SOx over NOxxe2x80x9d means that the SOx traps are sufficiently selective to absorb SOx over NOx so that the SOx traps will not be saturated with nitrate salts in the lean mode and consequently lose their SOx-trap capacity. In some cases, SOx trap materials can only form stable sulfate but not nitrate. For example, Mg, Mn, Cu, or Ni can selectively absorb SOx over NOx at 350xc2x0 C., respectively.
The term xe2x80x9csorbxe2x80x9d means to effect sorption.
The term xe2x80x9cstoichiometric/richxe2x80x9d mode or operation of treatment means that the gaseous stream being treated refers collectively to the stoichiometric and rich operating conditions of the gas stream.
The abbreviation xe2x80x9cTOSxe2x80x9d means time on stream.
The term xe2x80x9cwashcoatxe2x80x9d has its usual meaning in the art of a thin, adherent coating of a catalytic or other material applied to a refractory carrier material, such as a honeycomb-type carrier member, which is sufficiently porous to permit the passage therethrough of the gas stream being treated.
Other aspects of the invention are disclosed in the following detailed description of embodiments of the invention.